Overheat protector and protection methodology for electrodynamic loudspeakers
The present invention relates in one aspect to a voice coil temperature protector for electrodynamic loudspeakers. The voice coil temperature protector comprises an audio signal input for receipt of an audio signal supplied by an audio signal source and a probe signal source for generation of a low-frequency probe signal. A signal combiner is configured to combine the audio signal with the low-frequency probe signal to provide a composite loudspeaker drive signal comprising an audio signal component and a probe signal component. The voice coil temperature protector comprises a current detector configured for detecting a level of a probe current component flowing through the voice coil in response to the composite loudspeaker drive signal and a current comparator which is configured to comparing the detected level of the probe current component with a predetermined probe current threshold. The predetermined probe current threshold corresponds to a predetermined voice coil temperature via a known temperature dependency of a voice coil resistance. The voice coil temperature protector further comprises a signal controller configured for attenuating a level of the audio signal in response to the probe current component falls below the predetermined probe current threshold.
1. A method, comprising steps of:
adding a probe signal to a received speaker signal to generate a composite drive signal,
applying the composite drive signal to a voice coil of a loudspeaker,
detecting a voice coil current from the voice coil in response to the applied composite drive signal;
extracting, from the detected voice coil current, a level of probe signal current that corresponds to the probe signal portion of the composite drive signal,
comparing the extracted level of the probe signal current to a threshold corresponding to a predetermined thermal state of the speaker, and
attenuating a level of the speaker signal as applied to the loudspeaker based upon the comparison.
2. The method of claim 1 , wherein the attenuating comprises attenuating a level of the speaker signal within a predetermined sub band of the speaker signal.
3. The method of claim 1 , wherein the probe signal has a frequency at least five times smaller than a fundamental resonance frequency of the loudspeaker.
4. The method of claim 1 , wherein the probe signal has a frequency that is within a substantially flat impedance frequency range of the loudspeaker.
5. The method of claim 1 , wherein the probe signal has a period less than half a thermal time constant of the loudspeaker.
6. The method of claim 1 , wherein the probe signal, when active, has uniform amplitude.
7. The method of claim 1 , wherein the probe signal has an amplitude that varies with variations of the received speaker signal.
8. The method of claim 1 , further comprising when the comparison indicates the loudspeaker is operating within its thermal limits, disabling the probe signal.
9. The method of claim 1 , wherein the probe signal is a sine wave.
10. The method of claim 1 , wherein the probe signal is a noise signal.
11. The method of claim 1 , further comprising, prior to the adding:
detecting a level of the received speaker signal;
setting a level of the probe signal to a first level if the level of the received speaker signal exceeds a threshold; and
setting the level of the probe signal to a second level, smaller than the first level, if the level of the received speaker signal is below the threshold.
12. The method of claim 11 , wherein the detecting comprises detecting the level of the received speaker signal over a predetermined frequency sub-band.
13. A speaker monitor system, comprising:
a probe signal source configured to provide a probe signal;
a signal combiner having inputs for a speaker signal and for the probe signal from the probe signal source;
an amplifier having an input coupled to the signal combiner and an output for connection to a voice coil of a loudspeaker;
a detector having an input for a return signal from the voice coil of the loudspeaker, the detector configured to detect a portion of the return signal attributed to the probe signal;
a comparator having a first input configured to receive, from the detector, the detected portion of the return signal attributed to the probe signal and a second input configured to receive a threshold signal, the comparator configured to provide an output indicative of a relationship between the detected portion of the return signal attributed to the probe signal and the threshold signal; and
a controller configured to update a speaker signal gain based on the output of the comparator.
14. The system of claim 13 , wherein the controller attenuates a level of the speaker signal in response to the output of the comparator.
15. The system of claim 13 , wherein the detector comprises a bandpass filter.
16. The system of claim 13 , wherein the detector comprises a current sensor provided in a current path of the return signal, and an analog to digital converter having an input coupled to the current sensor.
17. The system of claim 13 , wherein the detector comprises a resistor provided in a current path of the return signal.
18. The system of claim 13 , wherein the detector comprises a current mirror provided in a current path of the return signal.
19. The system of claim 13 , wherein the controller attenuates a level of the speaker signal in a sub band of the speaker signal.
20. The system of claim 13 , wherein the probe signal source comprises a sine wave generator.
21. The system of claim 20 , further comprising the loudspeaker, wherein the sine wave has a frequency at least five times smaller than a fundamental resonance frequency of the loudspeaker.
22. The system of claim 13 , wherein the probe signal source comprises a noise generator.
23. A method comprising:
concurrently applying a loudspeaker drive signal and a probe signal to a voice coil of a loudspeaker, the loudspeaker drive signal including audible signal information and the probe signal including substantially inaudible, low-frequency signal information;
detecting a voice coil current signal from the voice coil in response to the concurrently applied loudspeaker drive signal and probe signal;
extracting, from the detected voice coil current signal, a probe current signal that corresponds to the applied probe signal; and
selectively attenuating the loudspeaker drive signal based on a level of the extracted probe current signal.
24. The method of claim 23 , wherein the concurrently applying the loudspeaker drive signal and the probe signal to the voice coil includes applying a probe signal that has a frequency that is at least five times smaller than a fundamental resonance frequency of the loudspeaker.
25. The method of claim 23 , wherein the concurrently applying the loudspeaker drive signal and the probe signal to the voice coil includes applying a probe signal that has a frequency that is within a substantially flat impedance frequency range of the loudspeaker.
26. The method of claim 23 , wherein the selectively attenuating the loudspeaker drive signal is based on a result of a comparison of the level of the extracted probe current signal and a specified threshold, the specified threshold determined based on a known temperature dependency of a resistance of the voice coil.
27. The method of claim 23 , wherein the probe signal includes substantially inaudible signal information between about 0.25 Hz and 20 Hz.
28. The method of claim 1 , wherein the adding the probe signal to the received speaker signal includes adding an AC probe signal having a frequency between about 0.25 Hz and 20 Hz to the received speaker signal.
29. The speaker monitor system of claim 13 , wherein the probe signal source is configured to provide an AC probe signal having a frequency between about 0.25 Hz and 20 Hz.